Proper memory function requires encoding of a memory during learning, consolidation of the memory in the hours and days that follow, and retrieval of the learned content during testing.
Memory consolidation is the process whereby the brain transfers memories to long-term storage. Consolidation of memories occurs primarily during sleep. Deep, or ‘slow-wave’ sleep (SWS), is particularly important for consolidating long-term memories. Recent advances in the fields of neurobiology, psychology, and sleep research have characterized the important relationship between sleep and memory.
Sleep is required for normal memory consolidation and reduced sleep quality or quantity disrupts memory function. In people, memory and other higher cognitive functions can be improved by increasing sleep quantity or sleep quality. Intensive training or learning causes an increase in the amount of SWS sleep during a subsequent night, suggesting that this phase of sleep is required for memories to be consolidated. In rodents, neurobiological studies have shown that patterns of activity among neurons in the hippocampus, a key brain region for memory, occur in a predictable and sequential pattern when a rodent is exploring a maze or other environment. The spatial memory represented by this experience is thought to be consolidated during sleep. Electrophysiological recordings during SWS have been used to identify ‘replay’ of the patterns of neural activity observed during previous experience, suggesting that replay is an important mechanism for consolidation of memories to long-term storage. Interruption of replay during sleep by electrical stimulation disrupts memory formation.
There are several phases of sleep that occur in a repeated cycle. Sleep phases can be identified by differences in brain activity and physiology, including variation in heart rate, body temperature, and arousal threshold. In humans, sleep is generally described according to a cycle in which rapid eye movement (REM) sleep is followed by non-REM sleep that generally proceeds sequentially through phases S1, S2, S3, and S4. Phases S1 and S2 are generally referred to as light sleep, and phases S3 and S4 are generally referred to as deep or ‘slow-wave’ sleep (SWS).
Normal cognitive function requires sufficient and well-structured sleep. Cognitive impairment due to sleep abnormalities occurs in individuals with neurodevelopmental disorders such as Down syndrome, neurodegenerative disorders such as Alzheimer's disease, various forms of insomnia, sleep apnea, and other pathological conditions. Similarly, reduced memory function unrelated to disease occurs with normal aging, overnight shift work, drug or alcohol use, and other causes of sleep impairment or sleep disruption. For these various forms of cognitive dysfunction, strategies to alleviate or mitigate cognitive deficits with pharmaceutical, educational, and behavioral interventions have received significant attention but have not adequately addressed cognitive deficits. New methods for improving the lives of those with intellectual disabilities, age-related cognitive decline, and other forms of learning disability by improving memory and cognitive function are desired. Moreover, healthy, typically-developed students of all ages would benefit from a method for enhancing memory consolidation and thus long-term memory retention.
The systems, methods and devices described herein may relate to augmenting or disrupting memory consolidation. These systems, methods and devices may allow the application of techniques to improve learning and memory non-invasively and without drugs and may engage memory consolidation processes that are active during sleep.
Although there is some academic work examining the presentation of sensory stimulus cues during sleep, this work has, to date, not been applied to a home setting in a manner that allows application of these techniques by an individual user. For example, in the first publication to report this effect, contextual presentations of olfactory cues during a prior learning event, when re-exposed during slow-wave sleep, were shown to improve the retention of memories formed during the learning event (Rasch, B., Büchel, C., Gais, S., and Born, J., 2007, Odor cues during slow-wave sleep prompt declarative memory consolidation. Science, 315, 1426-1429). In the simplest form of this technique, memory may be enhanced by (1) pairing learning with a sound or smell, (2) monitoring sleep during a subsequent night's sleep or nap, (3) detecting slow-wave (deep) sleep, and (4) re-presenting the sensory cue. Published studies of controlled, clinical studies of this technique have reported up to about 30% improvements in memory in healthy young adults (Diekelmann, S., Büchel, C., Born, J., and Rasch, B., 2011, Labile or stable: opposing consequences for memory when reactivated during waking and sleep. Nature neuroscience). Unfortunately, these studies provide little guidance on the application of these results in a home or user-applied setting, outside of a controlled laboratory setting.
Further, additional research has shown that misapplication of these techniques may lead instead to a decrease in memory. In contrast to the studies that reported enhanced memory after sensory re-presentation during sleep, re-presenting the sensory stimulus from training to the subject during wakefulness leads to a reduction in memory performance (Diekelmann et al., 2011). This finding also suggests methods for abolishing undesired memories such as those associated with traumatic events that lead to post-traumatic stress disorder (PTSD). Memory re-consolidation occurs when a memory that has been successfully stored in long-term memory is recalled. Maintenance of the memory in long-term memory after this event of memory recall requires active neurobiological processes to re-consolidate the stored memory trace. Accordingly, methods that selectively disrupt memories that are maladaptive, related to psychiatric conditions, or otherwise unwanted would be of great benefit to many.
Memory consolidation can also be reduced by disrupting sleep or by depriving a subject of sleep. Studies in humans and animals have shown that sleep deprivation or disruption of sleep after a training event lead to reduced memory performance.